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In 2014, a national measles outbreak was linked to Disneyland, where at least 39 cases were traced to direct exposure to the disease at the park. Over 170 people in 21 states became infected. In California, 88 percent of the measles patients were either unvaccinated or had unknown or undocumented vaccination status. This caused the largest US measles outbreak in decades, with 667 confirmed cases.

People have a variety of reasons for not vaccinating their children, but all philosophical and religious reasons fall into a category of vaccine exemptions called “Personal Belief Exemptions.” These exemptions are separate from medical exemptions. A study of California kindergarteners sought to figure out who are using non-medical exemptions to prevent vaccinations.

The study obtained publicly available Personal Belief Exemption (PBE) data for kindergarteners in the 2007-2008 and 2013-2014 school years for 6911 California schools from the California Department of Public Health’s “Immunization levels in child care and schools” pages. Income, education, and race of the parents, as well as school characteristics, were examined.

The following graph traces the percentage increase in PBEs according to a $25,000 increase in median household income up to $150,000. The data revealed that with each $25,000 interval, there was a nearly linear percentage increase in PBEs.

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